Method for producing a double-sided wiring board

ABSTRACT

The method for producing a printed wiring board comprising the steps of preparing a conductive substrate, forming an insulating layer on one surface of the said substrate, forming at least one via hole in the insulating layer, thermally curing the insulating layer, and reducing at least one oxidized layer formed on the other conductive surface of the substrate during the curing operation. Alternatively, the thermal cure may be accomplished in an atmosphere (e.g., reducing gas, inactive gas, or mixtures thereof) not conducive to oxide formation on metallized circuit surfaces.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 09/358,365,filed Jul. 21, 1999 now U.S. Pat. No. 6,571,467.

TECHNICAL FIELD

The present invention relates to a method for producing double-sidedwiring boards including surface layer printed wiring boards andmulti-layer double-sided wiring boards. More particularly, the inventionrelates to a method for forming wiring boards having high densitycircuits.

BACKGROUND OF THE INVENTION

Referencing FIGS. 7-11 of the drawings, a conventional double-sidedwiring board is produced as follows. First, a surface of copper foil 1with a thickness of a few to tens of micrometers is roughened as shownin FIG. 7(a). A double-sided copper-clad laminate can be used in placeof the copper foil 1. Then, as shown in FIG. 7(b), after one surface ofthe copper foil 1 is coated with photosensitive insulating resin 2, oneor more via holes 3 are formed at predetermined positions of the coatedphotosensitive insulating resin 2, and then the coated resin 2 isthermally cured. Next, as shown in FIG. 7(c), after the back surface ofthe copper foil 1 is coated with photosensitive insulating resin 4, oneor more via holes 5 are formed at predetermined positions of the coatedphotosensitive resin 4, and then the coated resin 4 is thermally cured.After surfaces of the thermally-cured photosensitive insulating resins 2and 4 on both surfaces of the copper foil 1 are smoothened and roughenedas shown in FIG. 7(d), these surfaces are plated with copper to formcircuit copper layers 6 and 7 as shown in FIG. 7(e).

In the above copper-plating process, the copper foil 1, on both surfacesof which the described photosensitive insulating resin layers 2 and 4are formed, is immersed in a copper plating solution. However, as shownin the enlarged view of FIG. 8, a portion of the bottom part of the backsurface of the copper foil 1, which is exposed by the via hole 5, isdissolved away, resulting in what is referred to as “haloing” (8)wherein an open portion is formed as shown in FIG. 8. Please see moreabout this below. As a result, the via hole 5 on which this haloing isgenerated is poorly plated with copper. In order to solve this problem,the thicknesses of the copper layers 6 and 7, including the haloed part8, is increased by a second copper plating, as shown in FIG. 9. Then,the circuits 9 are formed from the copper layers 6 and 7 by applying aknown photo-etching method. The result: a double-sided wiring board(10).

As stated, in the above-mentioned method, it is necessary to thicken thecopper-plated layers 6 and 7. However, doing so makes it difficult toform circuits 9 comprising fine lines. When the copper layers 6 and 7are formed on the photosensitive insulating resins 2 and 4 by platingthese resin surfaces with copper after via holes 3 and 5 are formed inthe photosensitive insulating resins on both surfaces of the copper foil1, immersion of this structure in acidic solution (used as a catalyst inthe chemical copper-plating process), results in the copper of theinterface between the insulating layer 4 and the copper foil 1 (on thevia hole) being dissolved. Thus, haloing occurs. As a result of suchhaloing, the contact surface between the copper layer 7 of the innersurface of the via hole 5 and the copper foil 1 of the bottom of the viahole 5 is not plated sufficiently with copper from only one copperplating. Thus, the thickness of the copper plating does not meet thestandard, and reliability of the product cannot be secured. To solvethis problem, a second copper plating process is implemented to secureenough thickness of copper plated layers 6 and 7 at the bottom of thevia hole 5. As a result, the copper-plated layers 6 and 7 on the surfacelayer are also thickened. As stated, such thick layers make highdensity, fine wiring extremely difficult.

When haloing occurs at the bottom (the top in FIG. 8) of the via holes5, adhesive strength between the insulating resin 4 and the copper foil1 is obviously diminished. Accordingly, as shown in FIG. 11, if spacesbetween via holes 5 are narrowed, the haloing 8 of via holes 5 mayoverlap each other, so that the adhesive strength between the insulatingresin 4 and the copper foil 1 in the haloed parts 8 is even furtherdiminished. In consequence; thermal stress in a soldering process maycause the peeling of copper foil 1 and insulating resin 4, as well asthe degradation of the resin's insulating properties.

For producing even greater density boas, both surfaces of the produceddouble-sided wiring board 10 are coated with photosensitive insulatingresins 11 and 12, preferably by the same process described above. Theresult: a double-sided wiring board 13 having circuits 9 in multi-layersas shown in FIG. 10. In such production, the resin 12 cannot be filledinto the via hole 5 on the back surface when the surface of the circuit7 on the back surface are coated with the photosensitive insulatingresin 12 because a “bubble” 14 may be generated inside of the hole (FIG.10). Therefore, as the number of via holes 5 or layers having the viaholes 5 is increased, a supplementary step of filling photosensitiveinsulating resin 12 to remove this open spacing (“bubble”) must beutilized. For this reason, it has been difficult to increase boardproductivity.

As the result of a preliminary investigation, a Japanese PatentPublication (No. 64-8479) related to haloing was located. According tothis publication, a chemical process for forming film of cupric oxideand then roughening said film surface is executed in order to improveadhesive strength between a copper foil circuit of a inner-layer printedwiring board comprising a copper-clad laminate board and a prepreg.However, in the chemical copper plating process, haloing may begenerated on the wall of the through-hole(s). To solve this problem,this Japanese Patent Publication mentions roughening thecopper-foil-circuit surface without damaging projections and formingrecesses on the surface (and spoiling affinity) by using a solution ofalkaline reducing agent to reduce cupric oxide film into cuprous oxideor copper metal.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a method for producinga double-sided wiring board wherein a multi-layered conductive layer canbe formed without haloing being generated.

Another object of the present invention is to provide a method forproducing a double-sided wiring board wherein a via hole can be coatedwith insulating resin without any bubbles being formed is the holeduring the process of making the board.

A further object of the present invention is to provide a method forproducing a double-sided wiring board. More than one conductive layercan be formed without any haloing being generated in the process ofproducing the board.

According to one aspect of the invention, there is defined a method forproducing a double-sided printed wiring board which comprises the stepsof providing a conductive substrate having first and second opposedconductive surfaces, forming a first insulating layer on the firstconductive surface of the substrate, forming at least one via hole inthe first insulating layer on the first conductive surface, thermallycuring the first insulating layer on the first conductive surface,resulting in the second conductive surface having a first oxidized layerthereon, removing the oxidized layer formed on the second conductivesurface of the conductive substrate, forming a second insulating layeron the second conductive surface of the conductive substrate from whichthe first oxidized layer is removed, forming at least one via hole inthe second insulating layer and forming conductive wiring on thesurfaces of both of the first and second insulating layers.

According to another aspect of the invention, there is defined a methodfor producing the double-sided wiring board comprising the steps ofproviding a conductive substrate having first and second opposedconductive surfaces, forming an insulating layer on the first conductivesurface of the substrate, forming at least one via hole in the firstinsulating layer on the first conductive surface, thermally curing thefirst insulating layer in a reducing gas, inactive gas, or a mixture ofthese gases so as to prevent an oxidized layer from being formed on thesecond conductive layer of the substrate during the thermally curingstep, forming a second insulating layer on the second conductive surfaceof the conductive substrate, forming at least one via hole in the secondinsulating layer and forming conductive wiring on the surfaces of boththe first and second insulating layers.

According to another aspect of the invention, there is defined a methodfor producing the multilayered double-sided wiring board comprising thesteps of forming a first insulating layer on the conductive wiring onone surface of the opposite surfaces of the double-sided wiring board,forming at least one via hole in the first insulating layer, thermallycuring the first insulating layer, resulting in the conductive wiring onthe other of the opposite surface having an oxidized layer thereon,removing the oxidized layer, forming a second insulating layer on thewiring on the other surface of the double-sided wiring board, forming atleast one via hole in the second insulating layer and forming wiring onthe surfaces of both the first and second insulating layers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) to 3(b) are sectional views showing the preferred steps formaking a double-sided wiring board according to the present invention;

FIGS. 4(a) to 5(b) are sectional views showing additional steps that canbe implemented to provide a double-sided wiring board of greater density(mom wiring layers) than that produced in FIGS. 1(a) to 3(b);

FIG. 6 is an enlarged sectional view to show a double-sided metal-cladlaminate board;.

FIGS. 7(a) to 10 are sectional views showing the aforedescribedconventional method for producing a double-sided wiring board; and

FIG. 11 is an illustration showing the relative spacing between twoadjacent via holes.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to the accompanying drawings, embodiments of the method forproducing a double-sided printed wiring board according to the presentinvention are described below.

As shown in FIG. 1(a), a metal layer 20 comprised of electricallyconductive material is used in the method for producing a double-sidedwiring board of the present invention, and surfaces 22 of the metallayer 20 are roughened as shown in FIG. 1(b). The metal layer 20 is asheet-like or foil-like layer having a thickness of a few to tens ofmicrometers, and is comprised of electrically good conductors such ascopper, aluminum, nickel or the like. Copper is preferably used. Topsurface 22 of metal layer 20 is roughened to prevent an insulating layer24 (FIG. 1(c)) applied over the top surface 22 from being removed (e.g.,peeled away) from said surface. Bottom surface 22 is also roughened, asseen in FIG. 1(b).

Normally, the step of roughening surfaces 22 of metal layer 20 isaccomplished by executing a reducing process after executing a chemicaloxidizing process. As a result of the chemically oxidizing process,extremely fine projections and recesses, each being of order ofsubmicrons, are formed on surfaces 22. If the metal layer 20 iscomprised of copper, an oxidizing agent such as sodium chlorite, sodiumhypochlorite, potassium chlorate, potassium perchlorate or potassiumpersulphate can be used for this purpose. The oxidizing process may beaccomplished by immersing the metal layer in a solution containing anyof the above-cited oxidizing agents, or by spraying the oxidizingsolution onto the metal layer. After the surfaces 22 of the metal layer20 are chemically oxidized, a reducing process is performed by immersingmetal layer 20 in an alkaline solution containing a reducing agent.However, it should be understood that metal layer 20 may be roughened bynot only applying such a chemically oxidizing process, but also byalternative means, such as acceptable electrical and physical methods.In such cases, a reducing process may not be necessary.

Next, an insulating layer 24 is formed on the top surface 22 of themetal layer 20 by coating the surface with insulating resin, as shown inFIG. 1(c). One or more via holes 26 are formed in the insulating layer24 at predetermined positions, and the insulating layer 24 is thenthermally cured. If a photosensitive insulating resin, such asphotosensitive epoxy resin, photosensitive acrylic resin orphotosensitive polyimide resin, is used for forming the insulating layer24, these one or more via holes 26 are formed at predetermined positionsprimarily by using a photo-etching process. If a thermosetting resin,such as epoxy resin, polyimide resin, or BT resin, is used for formingthe insulating layer 24, one or more via holes 26 may be formed atpredetermined positions by laser beam machining. As stated, after thevia holes 26 are formed, the insulating layer 24 is thermally cured(stabilized).

Following insulating layer thermal cure, the surfaces of the metal layer20, i.e., the bottom part of the via hole 26 and the back surface (thesurface on the other side) of the metal layer 20, which are not coveredwith the insulating layer 24, are further oxidized by heat to formoxidized layers 28. It is understood from FIG. 1(d) that layer 24shields much of the surface area on top surface 22 from said heat,leaving only that defined by via hole 28 as the area being subject tofurther treatment (here, oxidation) at this time. The oxidized layer 28formed on the back (bottom) surface of metal layer 20 may generatehaloing in the subsequent metal-plating process to be performed. Forthis reason, at least the back surface of the metal layer 20 is reducedas shown in FIG. 1(e). The reduction step is executed by immersing metallayer 20 having insulating layer 24 thereon in an alkaline solutioncontaining a reducing agent or by spraying, showering, or pouring thealkaline solution containing a reducing agent onto the back surface ofthe metal layer. If metal layer 20 is copper, an alkaline solutioncontaining a reducing agent, which is prepared by dissolving one or morekinds selected from the group of formaline, hypophosphorous acid, sodiumhypophosphite, hydrazine hydrochloride, hydrazine sulfate, hydrazinehydrate, or sodium borohydride in aqueous alkaline solution with a pH ofabout 7 to 13 (prepared by dissolving potassium hydroxide, sodiumhydroxide, or the like in water) can be used.

In the above reduction step, the oxidized layer 28 can also be reducedby heating the metal layer 20 on which the insulating layer 24 islocated up to a predetermined temperature in a process using a reducinggas (such as hydrogen gas, carbon monoxide gas, or a mixture of saidgases). It is the most preferable that the reduction step be executed inan atmosphere comprised only of such reducing gas. However, heattreatment may also be accomplished in an atmosphere of reducing gas andinert gas, or in an atmosphere of a large volume of reducing gas and asmall volume of oxygen.

As shown in FIG. 2(a), a step of forming an insulating layer 30 isperformed by coating the reduced back surface of metal layer 20 withinsulating resin, and then, as shown in FIG. 2(b), forming one or morevia holes 32 at predetermined positions within the insulating layer.Laser beam machining is preferably used, as described above, to formthese lower holes 32. After these steps, insulating layer 30 isthermally cured. As a result, portions of both surfaces of metal layer20 that are exposed by the bottom portions of via holes 26 and 32 areoxidized. Therefore, a similar reducing step as described above foreliminating the oxidized layers is performed. It is also possible toprevent oxidation from even occurring, to thermally cure the layer 30 inan atmosphere of inert gas or reducing gas or a mixture of these gasesto prevent oxide formation.

In FIG. 2(c), a step of smoothing the surfaces of the cured insulatinglayers 24 and 30 is performed, preferably by mechanical grinding means,following which the exposed surfaces are roughened again. It should beunderstood that this step of smoothing surfaces of the insulating layers24 and 30 is not always necessary. For example, when the sectionedconfigurations of the via holes 26 and 32 are like bowls (as shown) oranother shape that promotes excellent plating, a smoothing step may notbe required. In the roughening step, the surfaces of the insulatinglayers 24 and 30 are etched. The purpose of roughening surfaces of theinsulating layers 24 and 30 is to increase adhesive strengths betweenthe insulating layers 24 and 30 and the wiring metal layers (see below)when the wiring metal layers are formed on the surfaces of theseinsulating layers, e.g., by plating. Therefore, if adhesive strengthsbetween the insulating layers 24 and 30 and the subsequent wiring metallayers are high enough, a step of roughening surfaces of the insulatinglayers 24 and 30 is not required.

Next, as shown in FIG. 3(a), wiring metal layers 34 and 36 are formed onthe surfaces of the insulating layers 24 and 30. In this process, themetal layer 20, having the insulating layers 24 and 30 on its opposedsurfaces, is immersed in a plating solution. Since the surface of themetal layer 20, particularly, a part of the metal layer 20 which isexposed by a via hole 32, is not oxidized, the metal layer 20 is notcorroded by the plating solution, so that no haloing will result.Therefore, a second copper plating process is not required, if thewiring metal layers 34 and 36 are formed that each have a requiredminimum thickness. Then, as shown in FIG. 3(b), the wiring metal layers34 and 36 are formed into circuits 38 and 40 by a photo-etching processto produce a double-sided wiring board 42.

As is clear from the above description, since the plated wiring metallayers 34 and 36 are very thin, the width of each of the circuits 38 and40 and the space therebetween can be narrowed, producing a high densitywiring board structure. Further, since no haloing occurs on the bottomof the via hole 32, spaces between via holes can also be narrowed.Therefore, using the teachings of this invention, it is possible toproduce a high density wiring board, while preventing the insulatinglayer 30 from being peeled from the underlying metal layer 20.

In FIGS. 4 and 5, another embodiment of the invention is illustrated. Asshown therein, the method for the present invention can be appliedeffectively to provide more wiring layers on both surfaces of thedouble-sided wiring board formed by the method described above withrespect to FIGS. 1-3. As shown in FIG. 4(a), an insulating layer 44 isformed on one surface of the double-sided printed wiring board 42 bycoating the upper surface of circuit 38 with insulating resin, and then,a via hole 46 is formed at a predetermined position using a conventionalmethod. Then the insulating layer 44 is thermally cured. As a result ofthe above thermal curing process, the surfaces of circuits 40 formed onthe other side of the surface of the double-sided printed wiring board42 are oxidized. For this reason, as in the above-mentioned embodiment,the oxidized surfaces of the circuits 40 are reduced by: (1) immersingthe double-sided printed wiring board 42 in an alkaline solutioncontaining a reducing agent; (2) by spraying, showering, or pouring thealkaline solution containing a reducing agent onto the circuits 40 onthe other side of the surface of the double-sided printed wiring board42; or (3) by heat treating the circuits 40 in a reducing gas. It isalso preferable that the exposed surfaces of the double-sided printedwiring board 42 be roughened by applying a conventional method prior toa step of forming the insulating layer 44 on the upper surface of thedouble-sided printed wiring board in FIG. 4(a).

Next, as shown in. FIG. 4(b), an insulating layer 48 is formed bycoating the bottom surfaces of circuits 40 with insulating resin. Thereduced circuits 40 provide good wettability and good adhesion to theinsulating resin, so that substantially no bubbles are generated in thevia hole 32. In addition, as in the above-mentioned embodiment, anothervia hole 50 is formed at a predetermined position of the insulatinglayer 48 by applying a conventional method, and then the insulatinglayer 48 is thermally cured. After this curing, as shown in FIG. 4(c),surfaces of both the cured insulating layers 44 and 48 are smoothened bymechanical grinding means or the like, and then both surfaces areroughened. Processes for smoothing and roughening surfaces of theinsulating layers 44 and 48 are not always required, for the reasonsstated hereinabove.

As shown in FIG. 5(a), wiring metal layers 52 and 54 are now formed onthe external surfaces of insulating layers 44 and 48. In this process,although the double-sided wiring board 42 is immersed in a platingsolution, since the surface of the circuit 40 which is exposed by thevia hole 50 is not oxidized, the circuit 40 is not corroded by theplating solution, so that no haloing can be generated. Therefore, asecond copper plating process is not required, because the wiring metallayers 52 and 54 are so formed that each has a required, minimumthickness. Further, as shown in FIG. 5(b), the produced metal layers 52and 54 are then formed into circuits 56 and 58 through a conventionalphoto-etching process, thus producing multilayered double-sided wiringboard 60. Further, by applying the same production method as describedearlier Above, it is possible to produce an even higher densitymultilayer double-sided printed wiring board comprising an even greaternumber of layers, using the multilayer double-sided printed wiring board60 as a base component, as board 42 served as a base for eventual board60.

In the embodiment described above, since no haloing is generated at thebottom part of the via hole(s) 50, the wiring metal layer 54 formed atthe bottom part of the via hole 50 can be securely connected to thecircuit 40, resulting in improved reliability of the final structure. Inaddition, as described earlier, spaces between adjacent holes can benarrowed. And, since the wiring metal layers 52 and 54 are very thin,the width of each of the circuits 56 and 58, and a space between thesecan be narrowed. It is, therefore, possible to produce high densitywiring. Furthermore, since substantially no bubbles are generated in thevia holes, additional processing is not required. Productivity is thusgreatly improved.

In the above embodiment, should an oxidized layer be formed on the metallayer 20 or the circuit 40 because of thermal treatment, the oxidizedlayer is reduced and a plating process can be executed. However, it maybe possible to prevent the generation of oxidized layers even in thisembodiment. In the production method according to the first embodimentdescribed above, after a step of roughening the surfaces 22 of the metallayer 20, a step of forming an insulating layer 24 is executed bycoating one of the surfaces 22 of the metal layer 20 with insulatingresin, and then, a further step of forming one or more via holes 26 atpredetermined positions of the insulating layer 24 is executed. Afterthat, a step of thermally curing the insulating layer 24 is executed.

In the thermal curing step, the metal layer 20 on which the insulatinglayer 24 is formed, is heat-treated in an atmosphere of inert gas,reduced gas, or a mixture of these gases so that the metal layer 20 isnot oxidized. As inert gas, such gas as nitrogen gas, carbon dioxidegas, or a mixture of these gases can be used. It is preferable that airinside a heating chamber be replaced with inert gas. Such reducing gasesas hydrogen gas, carbon monoxide gas or a mixture thereof can be used.It is preferable that a sufficient amount of reducing gas be included inthe atmosphere inside a heating chamber so that the metal layer 20 canbe reduced rather than oxidized. Further, it may also be practicablethat the metal layer 20 is heat-treated by substituting the air insidethe heating chamber with a mixture of inert gas and reducing gas. In anycase, the surface of the metal layer 20 which is not covered with theinsulating layer 24 cannot be oxidized by inert gas or reducing gas,when the insulating layer 24 is thermally cured. The heat-treated metallayer 20 is then cooled down in inert gas or reducing gas to atemperature at which the metal layer 20 cannot be oxidized, even when itis directly contacted with oxygen, and then the metal layer 20 is takenout of the heating chamber.

Next, a step of forming the insulating layer 30 by coating the othersurface of the metal layer 20 with insulating resin is performed. Afterthat, a step of forming a via hole 32 in the insulating layer 30 isexecuted by applying a conventional method, and then a step of thermallycuring the insulating layer 30 is executed. In this thermal treatment,as in the above embodiment, the metal layer 20 may be heat treated ininert gas, reducing gas, or a mixture of these gases. However, haloingcannot be generated even when only the bottom part of the via hole 32surrounded by the insulating layer 30 is oxidized. Therefore, the metallayer 20 can be heat-treated in the air.

As shown in FIG. 2(c), the surfaces of the insulating layers on thecured both surfaces 24 and 30 were smoothened by mechanical grindingmeans, and then roughened. Wiring metal layers 34 and 36 were thenformed on the surfaces of the insulating layers 24 and 30, by a platingprocess. In this step, the metal layer 20, on which the insulatinglayers 24 and 30 are formed, is immersed in a plating solution. However,since the metal layer 20, particularly, a part of the metal layer 20which is exposed by the via hole 32, is not oxidized, the metal layer 20is not corroded by the plating solution and, thus, no haloing can begenerated. Then, the desired wiring metal layers 34 and 36 were formedrespectively into the circuits 38 and 40 by photo-etching or the like toproduce a double-sided wiring board 42 as seen in FIG. 3(b).

Further, the production method according to the above-described fistembodiment can achieve the same result as the production methodaccording to the second embodiment, also described above. Since areduction process is not required after a process for thermally curingthe insulating layers in the second embodiment, productivity can beimproved and production costs can be lowered. In a case where amultilayered double-sided wiring board with more layers is formed, e.g.,by using a double-sided wiring board 42 as a base, the insulating layerscan be thermally cured in inert gas, reducing gas, or a mixture of thesegases, as in the above-mentioned embodiment. In this case, the sameresult as that of the above embodiment can be achieved.

In the above embodiments, sheet- or foil-like metal layers eachcomprised of a singular metal may be used as the metal layer 20.However, as shown in FIG. 6, in place of such a single metal layer 20, adouble-sided metal-clad laminate 66 wherein conductive layers 64 arelaminated on both surfaces of an insulating layer 62 can be used.Generally, as a layer 62, an insulating resin with excellent mechanicalproperty as polyimide resin or polyamide resin is preferred. As aconductive layer 64, copper, silver, or aluminum can be used. Thedouble-sided metal-clad laminate 66 is produced by cladding these metalfoils onto both opposing surfaces of the interim resin layer 62 or byevaporating these metals onto both surfaces of the resin layer.

If layer 62 is an insulating layer, it is preferable to form and platethrough holes in order to electrically connect the conductive layers 64on both surfaces of the resin layer 62. It is also practicable toelectrically connect the conductive layers 64 by forming the resin layer67 with electrically conductive resin. In any case, a double-sidedmetal-clad laminate board 66 can also be used in the same manner as theabove-referred metal layer 20.

The method for producing a double-sided wiring board related to thepresent invention makes it possible to prevent haloing from beinggenerated in the metal plating process. For this reason, a relativelythin metal plating can be achieved. As a consequence, it is possible toform a thinner circuit and to narrow the spaces between via holes.

While there have been shown and described what are at present thepreferred embodiments of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention as defined bythe appended claims. For example, an alkaline solution containing areducing agent may be selected, depending on the metal used for theboard's metal layers (or circuits). In addition, reducing gases may beselected according to the kind of metal used for such metal layers orcircuits. The invention is understandably not limited to the specificmaterials described herein.

What is claimed is:
 1. A method for producing a double-sided wiringboard comprising the steps of: providing a conductive substrate havingfirst and second opposed conductive surfaces; forming an insulatinglayer on said first conductive surface of said substrate; forming atleast one via hole in said first insulating layer on said firstconductive surface; thermally curing said first insulating layer in areducing gas, inactive gas, or a mixture of these gases so as to preventan oxidized layer from being formed on said second conductive layer ofsaid substrate during said thermally curing step; forming a secondinsulating layer on said second conductive surface of said conductivesubstrate; forming at least one via hole in said second insulatinglayer; and forming conductive wiring on the surfaces of both said firstand second insulating layers.
 2. The method as defined in claim 1wherein the gas in which the first insulating layer is cured is areducing gas.
 3. The method as defined in claim 1 wherein the gas inwhich the first insulating layer is cured is an inactive gas.
 4. Themethod as defined in claim 1 wherein the gas in which the firstinsulating layer is cured is a mixture of a reducing gas and an inactivegas.
 5. The method according to claim 1, further comprising the step ofthermally curing said insulating layer on said second conductive surfaceof said conductive substrate to form a second oxidized layer within saidsecond conductive surface exposed by said at least one via hole formedwithin said second insulating layer.
 6. The method as defined in claim 5wherein the gas in which the first insulating layer is cured is areducing gas.
 7. The method as defined in claim 5 wherein the gas inwhich the first insulating layer is cured is an inactive gas.
 8. Themethod as defined in claim 5 wherein the gas in which the firstinsulating layer is cured is a mixture of a reducing gas and an inactivegas.
 9. The method of claim 1 further including the step of rougheningsaid first conductive surface of said conductive substrate prior toforming said first insulating layer on said first conductive surface ofsaid conductive substrate.
 10. The method of claim 9 further includingthe step of smoothening said roughened first conductive surface of saidconductive substrate between the step of forming said at least one viahole in said second insulating layer on said second conductive surfaceand the step of forming said conductive wiring on the surfaces of bothsaid first and second insulating layers.
 11. The method of claim 1wherein said first insulating layer comprises a photosensitiveinsulating resin.